The present invention relates to optical lenses.
Basic optical properties such as high light transmittance, high refractive index, low residual double refraction after molding, and the like are required for geometrical optical lenses which are employed in various cameras such as common cameras, single use cameras, video cameras, and the like, optical pickup units such as CD, CD-ROM, CD-Video, MO, CD-R, DVD, and the like, as well as OA equipment such as copiers, printers, and the like. In addition, general properties such as high thermal stability, high mechanical strength as well as hardness, low water absorption, high weather resistance, solvent resistance, and the like are also required. Further, for production at low cost, excellent moldability is required.
However, conventional molded optical lenses employing, for example, polymethyl methacrylate, polycyclohexyl methacrylate, polystyrene, polycarbonate, poly-4-methylpentene, norbornane based polymers, polyurethane resins, and the like have suffered from drawbacks such as low thermal stability, variation in optical properties due to high water absorption, high double refraction, and the like. As a result, conventional lenses have not sufficiently met the requirements listed above.
Furthermore, plastic lenses prepared employing polydiethylene glycol bisallyl carbonate have exhibited a low refractive index, as well as heat resistance which is not as high as inorganic glass.
Furthermore, in recent years, while high density recording systems, employing CD-R, DVD, MO, and the like, have been increasingly investigated and have been put into practice, more enhancement in recording density has been attempted by shortening the wavelength of the recording light source. Many of the aforementioned polyolefin based, polycarbonate based, and acrylic based resin materials exhibit very low spectral transmittance in the region of ultraviolet rays and do not transmit any of said ultraviolet rays. In addition, these resins have problems in which their degradation is accelerated due to the breakage of the polymer chain bond constituting their structure by ultraviolet rays.
Namely, in the future high density light recording, optical lenses which have high spectral transmittance for the ultraviolet region as well as excellent moldability are highly required.
Further, the technique to produce a plastic lens comes to its limit in terms of the reduction of the manufacturing cost and the formation of a more miniature lens under a current circumstance.
That is, in a current manufacturing apparatus, a resin material is melted before the resin material is poured into a mold, and then the melted resin material is put into the mold with a pressure. For such the production, a thermally plasticizing apparatus is required. Further, since the temperature distribution of the resin in the mold needs a high accuracy, a high precise temperature control device is needed to conduct a temperature control such that the temperature deviation is kept within 1.degree. C. or less. Further, it is necessary to apply a pressure required for a lens performance equally to overall of the lens. In order to apply the pressure equally, it may be difficult to produce a large number of lenses at a time in a predetermined sized mold with a single set of the thermally plasticizing apparatus. Also, the more a number of lenses produced at a time becomes, the larger a force required to remove the produced lens from the mold becomes.
Under the above restriction, the current production is in the situation that it may be difficult to lower its production cost less than the current cost. Further, due to the necessity of the above equal pressure, if a diameter of a lens becomes smaller, it is necessary to make the sectional area of a gate to pour the resin smaller. In this case, when it is necessary to make the gate smaller, for example, less than 3 mm square, it may be difficult to obtain an uniform pressure with a reverse proportion to the size of the gate. Of course, since there is a physical limitation in making the gate smaller, the miniaturization of the lens comes almost its limit under a current circumstance.